I have answered this when emailed about it, but let me explain for the community in general: as with many online shopping carts, you can only apply one coupon or not apply any coupons when an item is on sale. So, when an item is on sale, the Buy Out is over-ridden by the 'Sale'.
Our sales are typically any where from a few hours long to a month long - we actually decided last week that no sale should last more than one month - so we'll be shortening any of the longer sales that are on the site.
We are also considering making it possible to remove coupons or sales in the cart, which is already possible with Kits - I'll let you know what the plans are when we decide.
LikeSunshineDust: what email did you send your question to? I never received an email from you about this - I just double-checked, so I would really like to know.